1. Field of the Invention
This invention relates to an antenna apparatus.
2. Description of the Prior Art
A coaxial type vertical antenna so-called a sleeve antenna used for a radio wave apparatus as an antenna apparatus is known. FIG. 3 is a cross-sectional side view of a prior art sleeve antenna. An antenna apparatus comprises a coaxial cable, a balun 107, and an antenna element 101 which is provided by extending the inner conductor 102 of the coaxial cable. The inner conductor 102 and an outer conductor 103 of the coaxial cable are connected to a radio wave circuit 105 at connection points 104. The inner conductor 102 transmits an electric power from the radio wave circuit 105 to the antenna element 101. Around the outer conductor 103 of the coaxial cable, a coaxial sleeve electrically connected to the outer conductor 103 at a connecting point 106 is provided as a balun 107. The balun 107 forms another coaxial structure with the outer conductor 103 and removes an unbalanced current flowing through the outer conductor 103. Numeral 108 denotes a case of the radio wave apparatus.
An operation of the above-mentioned prior art antenna apparatus will be described.
A length of the antenna element 101 is a quarter of a wavelength used and acts as a monopole antenna. A length of the balun 107 is adjusted to a quarter of a wavelength to provide a tuned condition at the desired frequency. An impedance of the outer conductor 103 viewed from the connection point 106 becomes infinite at an opposite end from the connection point 106, that is, that point shows an open condition, so that if a voltage is applied to that point, no current flows through the outer conductor 103. As the result, this prevents an unbalanced current from flowing through an earth of the radio wave circuit from the antenna side and prevents a deterioration of a characteristic of the radio wave circuit 105.